The purpose of this work is to uncover the genetic changes that occur when one animal species becomes two. In particular, our goal is to find the genes that cause hybrid inviability and sterility and to identify the population genetic forces that drive their divergence. Using genetic and molecular population genetic analysis in Drosophila, we will address the following questions: 1. How many genes cause hybrid lethality between good species? 2. Which particular genes cause hybrid lethality? 3. What is the nature of the molecular differences distinguishing species at these genes? 4. Was divergence at these genes driven by natural selection? 5) Does meiotic drive in species hybrids cause hybrid sterility? Our long-term goal is to understand how animal species, including human, arise by determining the genetic cause of speciation.